The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant ...elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3–1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone–Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone–Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties.
Density functional theory (DFT) based computational electrochemistry has the potential to serve as a tool with predictive power in the rational development and screening of electrocatalysts for ...renewable energy technologies. It is, however, of paramount importance that simulations are conducted rigorously at a level of theory that is sufficiently accurate in order to obtain physicochemically sensible results. Herein, we present a comparative study of the performance of the static climbing image nudged elastic band method (CI-NEB) vs. DFT based constrained molecular dynamics simulations with thermodynamic integration in estimating activation and reaction (free) energies of the Volmer-Heyrovský mechanism on a nitrogen doped carbon nanotube. Due to cancellation of errors within the CI-NEB calculations, static and dynamic activation barriers are observed to be surprisingly similar, while a substantial decrease in reaction energies is seen upon incorporation of solvent dynamics. This finding is attributed to two competing effects; (1) solvent reorganization that stabilizes the transition and, in particular, the product states with respect to the reactant state and (2) destabilizing entropic contributions due to solvent fluctuations. Our results highlight the importance of explicitly sampling the interfacial solvent dynamics when studying hydrogen evolution at solid-liquid interfaces.
The oxygen evolution reaction (OER) is the limiting factor in an electrolyzer and the oxygen reduction reaction (ORR) the limiting factor in a fuel cell. In OER, water is converted to O
2
and H
+
/e
...−
pairs, while in ORR the reverse process happens to form water. Both reactions and their efficiency are important enablers of a hydrogen economy where hydrogen will act as a fuel or energy storage medium. OER and ORR can both be described assuming a four-step electrochemical mechanism with coupled H
+
/e
−
transfers between four intermediates (
M-*
,
M-OH
,
M = O
,
M-OOH
,
M
= active metal site). Previously, it was shown for mixed metal-oxyhydroxides that an unstable
M-OOH
species can equilibrate to an
M-OO
species and a hydrogenated acceptor site (
M-OOH/eq
), enabling a bifunctional mechanism. Within OER, the presence of Fe within a nickel-oxyhydroxide (NiOOH) acceptor site was found to be beneficial to lower the required overpotential (Vandichel et al. in Chemcatchem 12(5):1436–1442, 2020). In this work, we present the first proof-of-concept study of various possible mechanisms (standard and bifunctional ones) for OER and ORR, i.e. we include now the active edge sites and hydrogen acceptor sites in the same model system. Furthermore, we consider water as solvent to describe the equilibration of the
M-OOH
species to
M-OOH/eq
, a crucial step that enables a bifunctional route to be operative. Additionally, different single Fe-dopant positions in an exfoliated NiOOH model are considered and four different reaction schemes are studied for OER and the reverse ORR process. The results are relevant in alkaline conditions, where the studied model systems are stable. Certain Fe-dopant positions result in active Ni-edge sites with very low overpotentials provided water is present within the model system.
Graphic Abstract
Constrained density functional theory (CDFT) is a versatile tool for probing the kinetics of electron transfer (ET) reactions. In this work, we present a well-scaling parallel CDFT implementation ...relying on a mixed basis set of Gaussian functions and plane waves, which has been specifically tailored to investigate condensed phase ET reactions using an explicit, quantum chemical representation of the solvent. The accuracy of our implementation is validated against previous theoretical results for predicting electronic couplings and charge transfer energies. Subsequently, we demonstrate the efficiency of our method by studying the intramolecular ET reaction of an organic mixed-valence compound in water using a CDFT based molecular dynamics simulation.
The structures of AgCu, AgNi, and AgCo nanoalloys with icosahedral geometry have been computationally studied by a combination of atomistic and density-functional theory (DFT) calculations, for sizes ...up to about 1400 atoms. These nanoalloys preferentially assume core–shell chemical ordering, with Ag in the shell. These core–shell nanoparticles can have either centered or off-center cores; they can have an atomic vacancy in their central site or present different arrangements of the Ag shell. Here we compare these different icosahedral motifs and determine the factors influencing their stability by means of a local strain analysis. The calculations find that off-center cores are favorable for sufficiently large core sizes and that the central vacancy is favorable in pure Ag clusters but not in binary clusters with cores of small size. A quite good agreement between atomistic and DFT calculations is found in most cases, with some discrepancy only for pentakis-dodecahedral structures. Our results support the accuracy of the atomistic model. Spin structure and charge transfer in the nanoparticles are also analyzed.
A protocol for the accurate computation of electron transfer (ET) potentials from
ab initio
and density functional theory (DFT) calculations is described. The method relies on experimental p
K
a
...values, which can be measured accurately, to compute a computational setup dependent effective absolute potential. The effective absolute potentials calculated using this protocol display strong variations between the different computational setups and deviate in several cases significantly from the "generally accepted" value of 4.28 V. The most accurate estimate, obtained from CCSD(T)/aug-ccpvqz, indicates an absolute potential of 4.14 V for the normal hydrogen electrode (nhe) in water. Using the effective absolute potential in combination with CCSD(T) and a moderately sized basis, we are able to predict ET potentials accurately for a test set of small organic molecules (
σ
= 0.13 V). Similarly we find the effective absolute potential method to perform equally good or better for all considered DFT functionals compared to using one of the literature values for the absolute potential. For, M06-2X, which comprises the most accurate DFT method, standard deviation of 0.18 V is obtained. This improved performance is a result of using the most appropriate effective absolute potential for a given method.
A method to predict electron transfer potentials from first principles using the experimental p
K
a
as reference is shown.
Efficient hydrogen evolution reaction (HER) through effective and inexpensive electrocatalysts is a valuable approach for clean and renewable energy systems. Here, single‐shell carbon‐encapsulated ...iron nanoparticles (SCEINs) decorated on single‐walled carbon nanotubes (SWNTs) are introduced as a novel highly active and durable non‐noble‐metal catalyst for the HER. This catalyst exhibits catalytic properties superior to previously studied nonprecious materials and comparable to those of platinum. The SCEIN/SWNT is synthesized by a novel fast and low‐cost aerosol chemical vapor deposition method in a one‐step synthesis. In SCEINs the single carbon layer does not prevent desired access of the reactants to the vicinity of the iron nanoparticles but protects the active metallic core from oxidation. This finding opens new avenues for utilizing active transition metals such as iron in a wide range of applications.
Aerosol chemical vapor deposition is used to develop a highly active and durable non‐noble‐metal catalyst for the hydrogen evolution reaction by decorating single‐shell carbon‐encapsulated iron nanoparticles (SCEINs) on single‐walled carbon nanotubes (SWNTs). The catalyst exhibits catalytic properties comparable to those of platinum.
Oxygen evolution reaction (OER) via mixed metal oxy hydroxides M(O)(OH) may take place on a large variety of possible active sites on the actual catalyst. A single site computational description ...assumes a 4‐step electrochemical mechanism with coupled H+/e− transfers between 4 intermediates (M‐*, M‐OH, M=O, M‐OOH). We also consider bifunctional routes, in which an unstable M‐OOH species converts via a proton shuttling pathway to a thermodynamically more favourable bare M‐* site, O2 and a hydrogenated acceptor site; the acceptor site takes up the proton forming a hydrogenated acceptor site after recombination with an electron from the catalyst material. Here, we combine pure metal γ‐M(O)(OH) edge sites (M=Fe, Co, Ni) with as proton‐acceptor sites different threefold coordinated oxygens on β‐(M,M’)(O)(OH) terraces (M,M’=Fe, Co, Ni). The acceptor sites on these terraces have of a M’2MO motif. Our combinatorial study results in a ranking of the bifunctional OER activity on a 3D‐volcano plot. By studying various bi‐ and tri‐metallic oxy hydroxide combinations, we show that their excellent experimental OER activity results from bifunctionality and provide a roadmap to construct innovative low overpotential OER catalysts.
Oxygen Evolution Reaction: The bifunctional route shows the way in boosting the oxygen evolution activity on mixed metal‐oxy‐hydroxides. Edge and Acceptor sites collaborate for the creation of low overpotential reaction routes.
Reactions of CO and O
2
on neutral and anionic Cu
20
clusters have been investigated by spin-polarized density functional theory. Three reaction mechanisms of CO oxidation are explored: reactions ...with atomic oxygen (dissociated O
2
) as well as reactions with molecular oxygen, including Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The adsorption energies, reaction pathways, and reaction barriers for CO oxidation are calculated systematically. The anionic Cu
20
−
cluster can adsorb CO and O
2
more strongly than the neutral counterpart due to the superatomic shell closing of 20 valence electrons which leaves one electron above the band gap. The activation of O
2
molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu
20
and Cu
20
−
clusters, when O
2
is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu
20
. In general, CO oxidation occurs more readily on the anionic Cu
20
−
(effective reaction barriers 0.1-0.3 eV) than on the neutral Cu
20
cluster (0.3-0.5 eV). Moreover, Cu
20
−
exhibits enhanced binding for CO
2
. From the analysis of the reverse direction of CO oxidation, it is observed that the transition of CO
2
to CO + O can occur on the Cu
20
−
cluster, which demonstrates that Cu clusters may serve as good catalyst for CO
2
chemistry.
The anionic Cu
20
−
cluster can activate O
2
molecule upon adsorption and CO oxidation proceeds efficiently with the dissociated O
2
.
PDF
